Randall C. Rowe

Potato Early Dying: Management Challenges in a Changing Production Environment

The potato (Solanum tuberosum L.) had its start in the Andean highlands of South America, where archeological evidence indicates it has been cultivated for at least 8,000 years (72). Today it is grown on all continents except Antarctica. Serving as a primary food source to indigenous people when the Spanish explorers arrived in Peru in the mid-1500s, the potato traveled the globe as a valuable staple, fueling armies, improving diets, and changing economies (Fig. 1). The importance of the potato goes beyond the feeding of nations, however. Today potato starch is used in the production of paper, adhesive, and textile goods, and in edible binding agents and low-fat food additives. The potato yields up a highly absorbent biodegradable material for use in disposable diapers. It provides starch products to keep oil well drilling bits smooth and to hold together the ingredients in lipsticks and cosmetic creams, and it provides a substance that works as a flocculation agent in water purification systems, replacing petroleumbased chemicals (33). China is the world’s largest producer of potatoes, followed by the Russian Federation (33). It is the most important vegetable crop in North America, where in 2000, nearly 690,000 ha were grown, with an average yield of 32.6 metric tons/ha and a farm gate value of over

Recent progress in understanding relationships betweenVerticillium species and subspecific groups

2.7 billion. The potato is subject to several diseases caused by root-infecting pathogens. Two of particular significance are the soilborne fungi Verticillium dahliae and V. alboatrum. The potato root is also a host to the root-lesion nematode Pratylenchus penetrans. The two fungal pathogens alone, or in conjunction with P. penetrans, cause a disease called potato early dying (PED), also known as early die, early maturity wilt, and Verticillium wilt (64). This disease is endemic in many potato production areas of the United States, particularly in the Midwest and Pacific Northwest. In fields with a long history of potato production, it is a consistent yield constraint that requires intensive management. On land new to potato production, it may be absent in the first few years the crop is grown, but it almost invariably develops over time, requiring implementation of control measures to maintain high yields. In the last decade, the North American potato industry has undergone considerable changes. Production is increasingly being

Recovery of Verticillium dahliae from North American Certified Seed Potatoes and Characterization of Strains by Vegetative Compatibility and Aggressiveness

Improved understanding of the genetic diversity within fungi in the genusVerticillium has resulted from recent studies based on vegetative compatibility analysis and several techniques of molecular biology. Although the method used to identify vegetative compatibility groups (VCGs) does affect the results, vegetative compatibility appears to be a stable characteristic among isolates. Fairly low VCG diversity has been detected withinV. dahliae andV. albo-atrum using nitrate non-utilizing mutants. VCGs do not appear to be related to pathogenicity to particular host species, with the exception ofV. albo-atrum on alfalfa. However, there is some correlation with virulence on certain hosts and with the ability ofV. dahliae to interact with root-lesion nematodes. Studies based on DNA analysis indicate thatV. dahliae andV. albo-atrum are closely related but separate species. Restriction fragment length polymorphism (RFLP) studies have identified several subspecific groups withinV. dahliae, including two non-host-adapted groups and two that are host-adapted. They also have confirmed that alfalfa strains ofV. albo-atrum are a distinct subgroup that is probably a separate population of clonal origin. Using polymerase chain reaction (PCR), a second non-host-adapted subgroup withinV. albo-atrum was identified that was previously unknown.

Incidence of Colletotrichum coccodes in certified potato seed tubers planted in Washington State

Samples of tubers collected from commercial potato seed lots produced across North America were assayed forVerticillium dahliae. V. dahliae was successfully isolated from 65 of 224 seed lots tested, a successful isolation rate of nearly 30%. Vegetative compatibility of the isolates was assessed through complementation tests using nitrate non-utilizing mutants. AllV. dahliae isolates belonged to vegetative compatibility group (VCG) 4. Of the 162 isolates recovered, 64% belonged to VCG 4A, 33% to VCG 4B, and 3% to VCG 4AB. All 39 of the isolates tested in the greenhouse on potato cv. Superior were pathogenic to potato. Disease symptoms developed earlier, were more severe, and plants died earlier when inoculated with VCG 4A compared to 4B isolates. As a group, AUSPC values were significantly higher (p=0.05) for VCG 4A than for 4B isolates. These data suggest that (1) commercial certified seed tubers from diverse locations are commonly infected withV. dahliae and thus may serve as primary sources of the pathogen; (2) potato isolates ofV. dahliae in North America belong to VCG 4A and 4B and these strains are widely distributed via seed tubers; and (3) VCG 4A and 4B are distinct pathotypes ofV. dahliae that vary in their aggressiveness to potato.

Detection, Quantification, and Vegetative Compatibility of Verticillium dahliae in Potato and Mint Production Soils in the Columbia Basin of Oregon and Washington

Incidence of Colletotrichum coccodes in lots of certified seed tubers planted in Washington state, originating from nine western and midwestern states in the United State and two provinces in Canada, ranged from 0 to 90% in 1994 and 0 to 53% in 1995. In 1994, significant interactions between state/province and cultivar, and between seed grower and cultivar, were evident. In 1995, incidence of C. coccodes in seed lots did not vary significantly among states and cultivars. C. coccodes was not isolated from nuclear seed tubers and incidence of infected tubers was higher with higher seed generations. The fungus was isolated from the tuber periderm and outer medulla tissues and isolation frequency was greater from tuber stem ends than from either bud ends or lateral sections. Significantly greater stem infections developed in plants grown from seed tubers in which C. coccodes had been detected than in plants grown from seed tubers from which C. coccodes had not been isolated. This study confirms that C. coccodes is distributed among potato-production areas within seed tubers, and that seed tuber infection increases the incidence of early-season plant infection.

Lesion nematode involvement in potato early dying disease

Soil samples from 87 fields intended for potato production in the Columbia Basin of Washington and Oregon and 51 fields intended for mint production in Washington were assayed on a semiselective medium to quantify populations of Verticillium dahliae. The pathogen was isolated from 77 (89%) of the fields intended for potato production and 41 (80%) of the fields intended for mint production. Population densities ranged from 0 to 169 propagules/g of air-dried soil in fields intended for potato production and 0 to 75 propagules/g of air-dried soil in fields intended for mint production. Isolates of V. dahliae were recovered from soil assay plates and pure cultures were prepared to provide a collection of isolates for vegetative compatibility analysis. Among fields intended for potato production from which isolates of the fungus were assigned to a vegetative compatibility group (VCG), 93% of the fields were found to contain one or more isolates assigned to VCG 4A, nine (23%) contained one or more isolates assigned to VCG4B, and only one (3%) contained any isolates assigned to VCG 2B. In the case of fields planned for mint production in 1999 from which isolates of the fungus were assigned to a VCG, 13 fields (81%) were found to contain one or more isolates assigned to VCG 4A, 7 (44%) contained one or more isolates assigned to VCG 4B, and 5 (31%) contained one or more isolates assigned to VCG 2B. VCG 4A isolates of V. dahliae are widespread and numerous, particularly following potato production, but cause only mild to moderate symptoms in mint; therefore, this pathotype is unlikely to seriously endanger subsequent plantings of mint. However, planting potato in a field recently used to produce mint may pose a significant risk to the potato crop if high populations of the VCG4A pathotype (highly aggressive to potato) predominate. Preplant assessment of soil populations of V. dahliae without regard for the relative populations of various pathotypes present in a particular sample may lead to information not fully useful in integrated pest management systems.

THE AMERICAN PHYTOPATHOLOGICAL SOCIETY

Ohio field studies in microplots in the past 6 years have demonstrated a synergistic interaction of the lesion nematodePratylenchus penetrans withVerticillium dahliae in potato early dying. Tests with various populations ofP. penetrans andV. dahliae, alone and in combinations, have shown that disease occurs when both pathogens are present at populations which individually have little or no effect. Yield loss is not necessarily associated with foliar symptom development, but occurs with the addition of high temperature stress during tuberization. Comparative studies with two other species ofPratylenchus commonly found in Ohio potato soils have shown thatP. crenatus does not interact withV. dahliae and thatP. scribneri interacts only slightly. Since all three species feed and reproduce well on potato,Verticillium interactions probably are not due to wounding. The involvement ofPratylenchus withV. dahliae in potato early dying is an important factor that must be considered in the development of future crop-loss predictive systems.ResumenEstudios de campo, en microparcelas, en Ohio han demostrado una interacción sinergética entre el nematodo de la lesión radicularPratylenchus penetrans yVerticillium dahliae en la muerte prematura de la papa (MPP). En pruebas con varias poblaciones deP. penetrans yV. dahliae, solos o en combinaciones, se ha demostrado que la enfermedad ocurre cuando ambos patógenos están presentes en densidades de población que separadamente tendrían poco o ningún efecto. La pérdida de rendimiento no está necesariamente asociada con el desarrollo de síntomas foliares, pero ocurre cuando se presenta estrés por temperatura alta durante la tuberización. En estudios comparativos con otras dos especies dePratylenchus comúnmente encontrados en suelos donde se cultiva papa en Ohio, se ha demostrado queP. crenatus no interactúa conV. dahliae y queP. scribneri interactúa levemente. Como las tres especies se alimentan y reproducen bien en papa, las interacciones conVerticillium probablemente no se deben a las lesiones. Las vinculaciones entrePratylenchus yV. dahliae en la MPP son un factor importante que debe ser considerado en el desarrollo futuro de sistemas para predecir pérdidas en el cultivo.